tests: fix leak in drm-smoke-test.c

Leak found running drm-smoke-test with ASan.

Do not forget to destroy the SHM buffer by the end of the test.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

tests/drm-smoke-test.c

@@ -67,6 +67,7 @@ TEST(drm_smoke) {
 		frame_callback_wait(client, &frame);
 	}
 
+	buffer_destroy(buffer);
 	client_destroy(client);
 }